Glass substrate

ABSTRACT

The present invention is aimed to provide a method for producing a glass substrate with a thickness of not more than 200 μm, which is satisfied with the quality required for a substrate on which a thin-film electric circuit is formed, and a sheet glass substrate obtained according to this method. The present invention is concerned with a method for producing a glass substrate having a sheet thickness of from 10 to 200 μm, including a forming step of forming a molten glass into a ribbon shape in accordance with a down draw method, an annealing step of annealing the glass ribbon, and a cutting step of cutting the glass ribbon to give a glass substrate, wherein an average cooling rate in a temperature range of from the (annealing point +200° C.) to the (annealing point +50° C.) is controlled to the range of from 300 to 2,500° C./min.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/636,474 (allowed), filed Sep. 21, 2012, which is a §371National Stage Application of PCT International Application No.PCT/JP2011/056700, filed Mar. 22, 2011, which claims priority toJP2010-065568, filed Mar. 23, 2010 and JP 2011-049763, filed Mar. 8,2011, which are incorporated herein in their entirety.

TECHNICAL FIELD

The present invention relates to a glass substrate on which a thin-filmelectric circuit is formed, in particular to a glass substrate which isused for flat panel displays and flexible displays such as liquidcrystal displays, organic EL displays, and the like.

BACKGROUND ART

Glass substrates which are used for a display application are generallyformed according to a float method, a down draw method represented by anoverflow down draw method, or the like.

The float method is a method of casting a molten glass onto molten tin(float bath) and stretching it in the horizontal direction to form theglass in a sheet form. According to the method, a glass ribbon is formedon the float bath, and the glass ribbon is then annealed (on-lineannealed) in a long annealing furnace. Accordingly, the glass substrateformed according to the float method is characterized by having a smallthermal shrinkage ratio.

However, the float method involves such disadvantages that it isdifficult to make the sheet thin, the glass substrate is required to bepolished to remove tin attached onto the glass surface, and the surfacequality of the substrate is lowered.

However, the float method involves such disadvantages that it isdifficult to make the sheet thin, the glass substrate is required to bepolished to remove tin attached onto the glass surface, and the surfacequality of the substrate is lowered.

On the other hand, the down draw method is a generic term for a formingmethod of drawing a glass in the vertical downward direction to form itin a sheet form, and a slot (slit) down draw method, an overflow downdraw method, and the like are known. For example, in the overflow downdraw method that is widely adopted, a molten glass is introduced intothe top of a trough-shaped refractory (forming body) having a nearlywedge-shaped cross section, and the glass is allowed to overflow outfrom the both side thereof to flow down along the side face, and the twostreams are joined together at the lower end of the refractory and drawndownward to form the glass in a sheet form. The down draw method isadvantageous in that a glass is easy to be formed into a thin sheet.

Furthermore, in the case of the overflow down draw method, since theglass surface does not come into contact with any other than air, thereis also such an advantage that a glass substrate having high surfacequality can be obtained even in an unpolished state.

CITED REFERENCES Patent Documents

Patent Document 1: JP-A-2008-105882

Patent Document 2: JP-A-2008-133174

SUMMARY OF THE INVENTION Problems that the Inventions is to Solve

In recent years, from the viewpoint of space-saving, thinning and weightreduction of flat panel displays such as liquid crystal displays,organic EL displays, etc. are progressing, and as an extension thereof,researches toward flexibilization of the panels are energeticallyadvanced. In addition, because of expansion of electronic paper, newdisplay applications (e.g., electronic book, electronic newspaper,electronic price tag, digital signage, etc.) are being developed, and arequirement for thin and bendable flexible displays expands.

In order to realize a flexible display, the development of a substratetechnology is indispensable. A substrate having not only suppleness butbarrier properties against oxygen and moisture, etc. is necessary. Asthe substrate having these characteristics, thin sheet glasses which aremade thin as films are regarded as promising. In particular, from theviewpoint of suppleness, glasses which are thinner than 200 μm aredesired. Under these circumstances, the development of a method forproducing a thin sheet glass by adopting the down draw method isadvanced (see, for example, Patent Documents 1 and 2).

Similar to the current flat panel displays, it is expected thatrequirements, such as high precision, high fineness, etc., will be alsoincreased for the flexible displays. In order to meet theserequirements, it is necessary to make a pattern of a thin-film electriccircuit finer, and it is thought that a requirement of the surfacequality for the substrate will increase more and more. Incidentally, ifa surface roughness of the substrate (local unevenness) is large, or asheet thickness of the substrate (overall unevenness) is not uniform, itis difficult to form a fine circuit pattern.

However, in the case where it is intended to form a thin sheet glass ofnot more than 200 μm according to the down drawn method, it is difficultto stably draw out the glass from the forming equipment, and it isdifficult to keep the uniformity of the sheet thickness. For thatreason, there was involved such a problem that the quality required forthe substrate on which a thin-film electric circuit is formed cannot besatisfied. In order to make the sheet thickness uniform, it is thoughtto conduct an off-line polishing treatment. However, it is verytechnically difficult to polish a glass substrate of not more than 200μm, and the production costs greatly increase.

An object of the present invention is to provide a method for producinga glass substrate of not more than 200 μm, which is satisfied with thequality required for a substrate on which a thin-film electric circuitis formed, and a thin sheet glass substrate obtained according to thismethod.

Means for Solving the Problems

As a result of extensive and intensive investigations, the presentinventors have found that the foregoing object can be achieved byregulating an average cooling rate of the glass in a temperature regionhigher than an annealing point to 300° C./min or more and proposed it asthe present invention.

That is, a method for producing a glass substrate of the presentinvention is a method for producing a glass substrate having a sheetthickness of from 10 to 200 μm, including a forming step of forming amolten glass into a ribbon shape in accordance with a down draw method,an annealing step of annealing the glass ribbon, and a cutting step ofcutting the glass ribbon to give a glass substrate, wherein an averagecooling rate in a temperature range of from the (annealing point +200°C.) to the (annealing point +50° C.) is controlled to the range of from300 to 2,500° C./min. Incidentally, the “annealing point” is atemperature at which the glass has a viscosity of 10¹³ dPa·s, and thiscan be measured based on the method according to ASTM C336-71. The“average cooling rate” means a rate obtained in such a manner that thetime in which a center portion of a glass ribbon in the sheet widthdirection passes through a prescribed temperature region is calculated,and a temperature difference (here, 150° C.) within this region isdivided by the time taken for the passing.

According to the foregoing constitution, a glass substrate having auniform sheet thickness and having small warpage and residual stress canbe obtained by regulating the average cooling rate in a temperatureregion higher than the annealing point to 300° C./min or more. Inaddition, since the glass is rapidly cooled to the annealing point, thetime (or distance) capable of being taken for the subsequent annealingcan be sufficiently secured. As a result, nonetheless a fictivetemperature is high, by adequately regulating the subsequent annealingcondition, it is possible to produce a glass substrate having a smallthermal shrinkage ratio.

Furthermore, in the present invention, it is preferable to regulate anaverage cooling rate of from the annealing point to the (annealing point−100° C.) to the range of from 10 to 300° C./min.

In the case of increasing a cooling rate of the glass in a temperatureregion higher than the annealing point to form a sheet glass having asheet thickness of not more than 200 μm, the fictive temperature of theglass is easy to become high. When the fictive temperature of the glassbecomes high, in general, the thermal shrinkage ratio tends to becomehigh. As a result, there is a possibility that the quality required as asubstrate for forming a thin-film electric circuit cannot be satisfied.Even in such case, when the foregoing constitution is adopted,nonetheless the sheet thickness is not more than 200 μm, it is possibleto obtain a glass substrate having a low thermal shrinkage ratio.

In the present invention, the down draw method is preferably an overflowdown draw method.

According to the foregoing constitution, it is possible to produce asubstrate for forming a thin-film electric circuit, in particular aglass substrate capable of being used as a substrate of a flexibledisplay, in a surface state at the time of forming as it is.Accordingly, it is possible to omit a polishing step, and thisconstitution is suitable as a method for producing a thin sheet which isdifficult to be polished.

In the present invention, it is preferable to use a glass comprisingfrom 50 to 70% of SiO₂, from 10 to 25% of Al₂O₃, from 1 to 15% of B₂O₃,from 0 to 10% of MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0to 15% of BaO, and from 0 to 5% of Na₂O in terms of percentage by mass.

According to the foregoing constitution, it is easy to select a glasscomposition having a high strain point and having a liquidus viscositysuitable for the overflow down draw method. In addition, it is possibleto make a glass composition which is excellent in variouscharacteristics required for display substrates, such as chemicalresistance, specific modulus, chemical durability, meltability, etc.

The glass substrate of the present invention is a glass substrate havinga sheet thickness of from 10 to 200 μm, and it is characterized in thata sheet thickness difference between a maximum sheet thickness and aminimum sheet thickness in the substrate is not more than 30 μm.Incidentally, the “sheet thickness difference between a maximum sheetthickness and a minimum sheet thickness in the substrate” means a valueobtained by measuring thickness variation along an arbitrary line acrossa glass substrate using a laser type thickness measuring device,determining a maximum thickness and a minimum thickness of the glasssubstrate, and then subtracting a value of the minimum sheet thicknessfrom a value of the maximum sheet thickness.

According to the foregoing constitution, since the substrate hasflexibility, it is possible to use the substrate for an application fora substrate of a flexible display, or the like. In addition, the sheetthickness difference necessary for the substrate on which a thin-filmelectric circuit is formed can be satisfied.

In the present invention, a residual stress value is preferably not morethan 2.5 nm. In the present invention, the “residual stress value” meansa retardation value measured using a stress meter according to anoptical heterodyne method.

According to the foregoing constitution, the distortion value necessaryfor the substrate on which a thin-film electric circuit is formed can besatisfied.

In the present invention, a warpage value is preferably not more than200 μm. Incidentally, in the present invention, the “warpage value”means a value measured by a warpage measurement system.

According to the foregoing constitution, the warpage value necessary forthe substrate on which a thin-film electric circuit is formed can besatisfied.

In the present invention, a thermal shrinkage ratio at the time ofheating from ordinary temperature at a rate of 5° C./min, keeping at450° C. for 10 hours, and then cooling at a rate of 5° C./min ispreferably less than 300 ppm. Incidentally, in the present invention,the “thermal shrinkage ratio” means a value obtained through themeasurement in the following manner. First of all, a strip sample of 160mm×30 mm is prepared as a sample for the measurement (FIG. 2( a)).Markings are given to the area around from 20 to 40 mm from each end ofthis strip sample in the long side direction with a #1000 waterproofabrasive paper, and the sample is divided into two pieces along thecenter line vertical to the markings (FIG. 2( b)). After one of thepieces is heat treated under prescribed conditions, the heat-treatedpiece and untreated piece are put in parallel (FIG. 2( c)), displacementof the markings (AL1 and AL2) are measured with a laser microscope, andthe thermal shrinkage ratio is calculated according to the followingequation.

Thermal shrinkage ratio [ppm]=(ΔL1 [μm]+ΔL2 [μm])/160×10⁻³

According to the foregoing constitution, there is brought such an effectthat even when the heat treatment is applied in the forming step of athin-film circuit pattern, a pattern displacement is hardly caused.

In the present invention, an average surface roughness Ra is preferablynot more than 0.3 nm. Incidentally, in the present invention, the“average surface roughness Ra” means a value measured according to amethod in conformity with the “FPD Glass Substrate Surface RoughnessMeasurement Method” in SEMI D7-94.

So far as the foregoing constitution can be directly achieved byadopting the overflow down draw method or the like, it is possible toomit the polishing step.

In the present invention, the substrate is preferably composed of aglass comprising from 50 to 70% of SiO₂, from 10 to 25% of Al₂O₃, from 1to 15% of B₂O₃, from 0 to 10% of MgO, from 0 to 15% of CaO, from 0 to15% of SrO, from 0 to 15% of BaO, and from 0 to 5% of Na₂O in terms ofpercentage by mass.

According to the foregoing constitution, since the glass has a highstrain point, and a liquidus viscosity suitable for the overflow downdraw method, a glass which is low in the thermal shrinkage ratio andexcellent in the surface quality can be obtained without being polished.

In the present invention, it is preferable to use the substrate as asubstrate for forming a thin-film electric circuit, in particular asubstrate of a flexible display.

According to the foregoing constitution, the characteristic features ofthe present invention that not only the sheet thickness is small, butthe surface quality is excellent can be made the best use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outline front view showing production equipment for a glasssubstrate in carrying out the present invention.

FIGS. 2( a) to 2(c) are explanatory views showing a method for thermalshrinkage ratio determination.

MODES FOR CARRYING OUT THE INVENTION

The method of the present invention is described in detail.

First of all, the method of the present invention includes a formingstep of forming a molten glass into a ribbon shape in accordance with adown draw method. In this forming step, it is important to regulateforming conditions such that a sheet thickness of the glass to befinally obtained is from 10 to 200 μm. The sheet thickness can beregulated by controlling a flow rate of the glass, a formingtemperature, a rate of drawing the glass (sheet drawing rate), and thelike. Incidentally, as for the forming conditions, it is preferable toregulate the sheet thickness of the glass to be obtained to from 10 to150 μm, in particular from 10 to 100 μm.

Though the forming method is not particularly limited so far as it isthe down draw method, it is preferable to adopt the overflow down drawmethod capable of producing a ribbon-shaped glass with a favorablesurface quality without conducting polishing. The reason why when theoverflow down draw method is adopted, a ribbon-shaped glass with afavorable surface quality can be produced resides in the matter that theside thereof serving as a surface of the ribbon does not come intocontact with any other than air and is formed in a free-surface state.Incidentally, the overflow down draw method is a method in which amolten glass is allowed to overflow out from the both side of aheat-resistant trough-shaped structure, and the overflown glasses aredrawn and formed downward while being joined together at a lower end ofthe trough-shaped structure, thereby producing a ribbon-shaped glass.The structure and material quality of the trough-shaped structure arenot particularly limited so far as the dimension or surface precision ofthe ribbon-shaped glass, or the quality required for a predeterminedapplication can be realized. In addition, for the downward drawing, anymethod for applying a force to the ribbon-shaped glass may be adopted.For example, a method in which the molten glass is drawn by rotatingheat-resistant rollers having a sufficiently large width in a state ofbeing brought into contact with the ribbon-shaped glass may be adopted;and a method in which the molten glass is drawn while bringing pluralpairs of heat-resistant rollers into contact with only around the bothend surfaces of the ribbon-shaped glass may be adopted.

Incidentally, in the present invention, in addition to the overflow downdraw method, various down draw methods can be adopted. For example, itis possible to adopt a slot down method, a redraw method, or the like.

The method of the present invention comprises an annealing step ofcooling the glass formed into a ribbon shape. In this step, in a processof cooling the high-temperature ribbon-shaped glass immediately afterforming, control of the sheet thickness, removal of the residual stressor warpage, reduction of the thermal shrinkage, and the like areconducted. In particular, the present invention is characterized bycontrolling the cooling rate to a specified rate in a temperature regionof the annealing point or higher at which the sheet thickness, residualstress, or warpage is greatly influenced. Specifically, an averagecooling rate in a temperature range of from the (annealing point +200°C.) to the (annealing point +50° C.) is controlled to the range of from300 to 2,500° C./min, preferably from 300 to 2,000° C./min, from 300 to1,500° C./min, from 400 to 1,000 ° C./min, from 500 to 900° C./min, andespecially preferably from 600 to 800° C./min. Incidentally, for thesake of convenience, the temperature range of from the (annealing point+200° C.) to the (annealing point +50° C.) is hereinafter referred to as“first annealing temperature region”.

Incidentally, the temperature of the glass can be determined by means ofnon-contact measurement with a pyrometer or contact measurementutilizing a thermocouple.

When the cooling rate of the first annealing temperature region is toolow, the shape of the glass sheet is not rapidly defined, and hence, itis difficult to make the sheet thickness uniform. In addition, the time(or distance) capable of being taken for the subsequent annealingbecomes short, and hence, the thermal shrinkage ratio becomes large. Onthe other hand, when the cooling rate of the first annealing temperatureregion is too high, the glass is rapidly cooled, and hence, anon-uniform, large residual stress is generated, resulting indeterioration of the warpage. In addition, the fictive temperature ofthe glass becomes too high, and therefore, even by regulating thesubsequent annealing conditions, it is difficult to sufficientlydecrease the thermal shrinkage ratio.

Incidentally, the fictive temperature is a temperature of a supercooledliquid having the same structure as a glass structure, and this is anindex of the structure of glass. Glass is low in viscosity and liquidusat a high temperature, and in this stage, the glass has an openstructure. Then, when the glass is cooled, the glass structure becomesdense and is frozen. This glass structure change occurs because theglass is likely to be in the most stable state at that temperature.However, when the cooling rate of glass is high, the glass structure isfrozen before it has a dense structure corresponding to thattemperature, so that the glass structure is frozen in a state of ahigh-temperature side. The temperature corresponding to the solidifiedglass structure is said to be a fictive temperature. When the fictivetemperature is higher, the glass structure is more open and therefore,the thermal shrinkage ratio becomes large. However, when the subsequentannealing is adequately conducted, it is possible to make the thermalshrinkage ratio small. In the case of carrying out the method of thepresent invention, the fictive temperature of the glass substrate iseasy to fall within the range of from the (annealing point +45° C.) tothe (annealing point +100° C.), in particular the range of from the(annealing point +45° C.) to the (annealing point +80° C.), and moreoverthe range of from the (annealing point +45° C.) to the (annealing point+60° C.). In the case of the method of the present invention, accordingto a fast cooling rate in the first annealing region, the time capableof being taken for the annealing in a temperature region of not higherthan the annealing point can be ensured long. Therefore, by adequatelyregulating the annealing conditions, nonetheless the fictive temperatureis high, a glass substrate having a practically acceptable thermalshrinkage ratio can be obtained.

The “fictive temperature” is a temperature determined as follows. Firstof all, the same glass piece as that in the thermal shrinkagedetermination is put into an electric furnace controlled at theannealing point temperature, and after one hour, the glass piece istaken out of the electric furnace and rapidly cooled on an aluminumplate, followed by measuring a thermal shrinkage ratio thereof. The sametreatment is carried out at the (annealing point +20° C.), the(annealing point +40° C.), and the (annealing point +60° C.),respectively, and a graph of a relationship between the treatmenttemperature and the thermal shrinkage ratio is prepared. A heattreatment temperature at which the thermal shrinkage ratio is 0 ppm isdetermined from a linear approximate curve of this graph, and this isdefined as the fictive temperature of glass.

Now, in the down draw method, in view of the relationship that anannealing furnace is provided just below the forming body, it isactually impossible to dispose a long annealing furnace like that in thefloat method. Accordingly, the annealing furnace is necessarily short.In other words, the cooling rate within the annealing furnace is fast,and a glass is frozen in a high temperature state, and therefore, it isdifficult to obtain a glass substrate having a small thermal shrinkageratio.

In liquid crystal displays or organic EL displays, a thin-film electriccircuit such as a thin film transistor (TFT) is formed on the surface ofa glass substrate. In this forming process, when the glass substrate isexposed to a high-temperature atmosphere, structural relaxation isadvanced, and its volume shrinks (thermally shrinks). When the glasssubstrate thermally shrinks in a forming step of a thin-film electriccircuit, the shape and dimension of the circuit pattern deviate from thedesigned values, whereby desired electric performances are notobtainable. For that reason, it is required that the thermal shrinkageof the substrate on which a thin-film electric circuit is formed issmall.

Then, in the method of the present invention, it is preferable toregulate an average cooling rate in a temperature range of from theannealing point to the (annealing point −100° C.), which is atemperature region subsequent to the first annealing temperature region,to the range of from 10 to 300° C./min. In particular, it is desirableto regulate the average cooling rate to the range of from 10 to 200°C./min, from 10 to 150° C./min, and from 50 to 150° C./min.Incidentally, for the sake of convenience, the temperature range of fromthe annealing point to the (annealing point −100° C.) is hereinafterreferred to as “second annealing temperature region”. The secondannealing temperature region is a temperature region at which thethermal shrinkage ratio is greatly influenced, and by passing throughthis region at the foregoing cooling rate, nonetheless the fictivetemperature is high, a glass substrate having a practically acceptablethermal shrinkage ratio can be produced. When the cooling rate in thisrange is too low, in the case of the present invention for forming aglass according to the down draw method, a glass melting apparatus or aforming furnace must be set at higher sites, so that there is a concernthat this brings about limitations from the standpoint of designing theequipment. On the other hand, when the cooling rate is too high, thetime capable of being taken for the annealing is short, and hence, as aresult, it becomes difficult to reduce the thermal shrinkage ratio.

Incidentally, in the method of the present invention, it is desirablethat in the annealing step, an average cooling rate in a temperatureregion positioning between the first annealing temperature region andthe second annealing temperature region, namely in a temperature rangeof from the (annealing point +50° C.) to the annealing point, is setlower than the cooling rate in the first annealing temperature regionand higher than the cooling rate in the second annealing temperatureregion. Incidentally, for the sake of convenience, the temperature rangeof from the (annealing point +50° C.) to the annealing point ishereinafter referred to as “intermediate annealing temperature region”.By setting the cooling rate in the intermediate annealing temperatureregion as described above, a change of the cooling rate from the firstannealing temperature region to the second annealing temperature regioncan be smoothly achieved.

The method of the present invention comprises a cutting step of cuttingthe ribbon-shaped glass after completion of the annealing into aprescribed length to form a glass substrate. The cutting as referred toherein is not limited to the case of cutting off the ribbon-shaped glassdirectly every sheet. That is, the cutting includes the case where theribbon-shaped glass is once wound up in a roll form and then subjectedto various processings such as rewinding, sheet width adjustment, filmcoating, etc., and thereafter, the ribbon-shaped glass is again drawnout and cut every sheet. For the cutting, various methods such as amethod for previously making a scribed line on a glass with a cutter ora laser light and then divided the glass, a method for fusing a glasswith a laser light, etc. can be adopted.

In the method of the present invention, it is desirable that the surfaceof the obtained glass substrate is not subjected to polishing. Namely,in a glass having a sheet thickness of from 10 to 200 μm, thepossibility of breakage during polishing process is very high.Accordingly, when polishing is applied, a production yield becomes low,and special equipment for preventing the breakage caused duringpolishing process is needed, and hence, the costs increase. Moreover,when polishing is conducted, the glass surface is scratched, and theoriginal strength of glass is impaired. Incidentally, in order to obtaina glass substrate having an excellent surface quality even withoutapplying polishing, an overflow down draw method may be adopted as theforming method. Incidentally, the “surface” as referred to in thisdescription means a translucent surface (or a main surface) of the glasssubstrate, and it is differentiated from an edge surface to whichpolishing is applied for the purpose of preventing cracking, etc.

In the method of the present invention, it is preferable to use a glasshaving a liquidus viscosity of 10^(4.5) dPa·s or more. In particular, inthe case of forming a glass according to the overflow down draw method,it is important that the liquidus viscosity of the glass is high.Specifically, the liquidus viscosity of the glass is preferably 10^(4.5)dPa·s or more, 10^(5.0) dPa·s or more, 10^(5.5) dPa·s or more, and 10⁶⁰dPa·s or more. Incidentally, the liquidus viscosity is a viscosity atthe temperature of precipitation of a crystal, and a composition havinga higher liquidus viscosity is more hardly devitrified at the time ofglass forming and is easier to be formed into a glass.

In the method of the present invention, it is preferable to use a glasshaving a strain point of 600° C. or higher. The stain point as referredto herein means a temperature at which the glass has a viscosity of10^(14.5) dPa·s. According to this constitution, it is easy to produce aglass substrate having a small thermal shrinkage ratio.

The method of the present invention can be applied to various glasses.For example, in the case of expecting the use for a liquid crystaldisplay, an organic EL display, etc., a glass comprising from 50 to 70%of SiO₂, from 10 to 25% of Al₂O₃, from 1 to 15% of B₂O₃, from 0 to 10%of MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0 to 15% ofBaO, and from 0 to 5% of Na₂O in terms of percentage by mass may beused. So far as the composition falls within this range, it is easy todesign a glass composition having a high strain point and having aliquidus viscosity suitable for down draw forming.

In the glass substrate obtained by the present invention, by adequatelyregulating the first annealing temperature region, it is possible toregulate a sheet thickness difference between a maximum sheet thicknessand a minimum sheet thickness in the substrate to not more than 30 μm,in particular not more than 25 μm, and moreover not more than 20 μm. Inthe case where the sheet thickness difference is too large, it isdifficult to conduct accurate patterning of an electrode, etc., andfaults such as disconnection or short circuit of a circuit electrode,etc. are easily caused.

In the glass substrate obtained by the present invention, by adequatelyregulating the first annealing temperature region, it is possible toregulate a residual stress value to not more than 2.5 nm, in particularnot more than 2.2 nm, and moreover not more than 2.0 nm. When theresidual stress value is too large, there are caused such faults that apattern deviates at the time of cutting the glass substrate; that in anapplication of liquid crystal display substrate, a homogenous image isnot obtained due to birefringence; and the like.

In the glass substrate obtained by the present invention, by adequatelyregulating the first annealing temperature region, it is possible toregulate a warpage value to not more than 200 μm, in particular not morethan 100 μm, and moreover not more than 80 μm. When the warpage value istoo large, it is difficult to conduct accurate patterning of anelectrode, etc., and faults such as disconnection or short circuit of acircuit electrode, etc. are easily caused.

In the glass substrate obtained by the present invention, a thermalshrinkage ratio at the time of heating from ordinary temperature at arate of 5° C./min, keeping at 450° C. for 10 hours, and then cooling ata rate of 5° C./min is easy to become less than 300 ppm. Since it ispreferable that the thermal shrinkage ratio of the glass is smaller, byadequately regulating the second annealing temperature region, it ispossible to control the thermal shrinkage ratio of glass to not morethan 250 ppm, moreover not more than 200 ppm, and in particular not morethan 100 ppm. When the thermal shrinkage ratio is too large, in the casewhere the glass substrate is used as a substrate for forming a thin-filmelectric circuit, the circuit pattern deviates from the expected design,and electric performances cannot be maintained.

In the glass substrate obtained by the present invention, by forming aglass substrate according to the overflow down draw method and omittingthe polishing step, it is possible to regulate an average surfaceroughness Ra to not more than 0.3 nm, in particular not more than 0.2nm. Incidentally, the average surface roughness of a glass to whichpolishing is applied exceeds 0.3 nm.

Next, the glass substrate of the present invention is described.

Various characteristic features of the glass substrate of the presentinvention, such as sheet thickness, sheet thickness difference,distortion value, warpage value, thermal shrinkage ratio, surfaceroughness, composition, etc., are those as already described, and adescription thereof is omitted herein. In addition, the glass substrateof the present invention can be produced according to the method of thepresent invention as described above.

Incidentally, in the glass sheet of the present invention, its sheetwidth is not particularly limited. The sheet width can be varied byregulating the length of a slot or the like from which a glass is drawnout in the case of the slot down draw method, or by regulating thelength of a forming body or the like in the case of the overflow downdraw method.

The glass substrate of the present invention can be used for variousapplications. For example, the glass substrate of the present inventioncan be used as a glass substrate on which a thin-film electric circuitis formed. Since the glass substrate of the present invention has auniform sheet thickness and has a small residual stress value or warpagevalue, the quality required for a substrate on which a thin-filmelectric circuit is formed can be satisfied. Furthermore, when thethermal shrinkage ratio is made small, the substrate hardly causesthermal shrinkage by the heat treatment in the forming step of athin-film electric circuit, and problems such as a displacement of thecircuit pattern, etc. can be easily avoided.

In addition, it is preferable to use the glass substrate of the presentinvention as a substrate for a flexible display. In view of the factthat the glass substrate of the present invention has a small sheetthickness, it has flexibility, and suppleness necessary as a flexibledisplay substrate can be obtained.

EXAMPLES

The present invention is hereunder described in detail by reference tothe accompanying drawings.

FIG. 1 is an outline front view showing production equipment for a glasssubstrate in carrying out the present invention. The productionequipment is for producing a glass substrate according to an overflowdown draw method, and it includes a forming furnace 1 having atrough-shaped forming body 11 and cooling rollers 12 therein in thisorder from the top thereof; an annealing furnace 2 disposed in a lowerportion of the forming furnace 1 and having heaters 21 and guide rollers22 therein; and a cooling section 3 and a cutting section 4 provided ina lower portion of the annealing furnace 2.

The trough-shaped forming body 11 has a nearly wedge-shaped crosssection and allows a molten glass G1 to be fed to overflow out from thetop thereof and fuse at the bottom thereof to form a glass ribbon G2.The annealing furnace 2 anneals the glass ribbon G2. In detail, in theinside of the annealing furnace 2, a plural number of the panel heaters21 are provided at the both side of the glass ribbon G2 facing to theglass ribbon G2. The heaters 21 are disposed in plural series and inplural rows in the conveyance direction (vertical direction) and in thesheet width direction (horizontal direction), and the temperaturethereof can be independently controlled. The cooling section 3thoroughly cools the annealed glass ribbon G2. The cutting section 4cuts the cooled glass ribbon G2 into a prescribed dimension. Inaddition, in the cutting section 4, a conveyance route for conveying aglass substrate G3 into a non-illustrated subsequent step (for example,a packing step, etc.) is separately provided.

Next, the production method for a glass substrate of the presentinvention using the foregoing production equipment is described.

In this production equipment, first of all, the molten glass G1 is fedto the top of the trough-shaped forming body 11 provided within theforming furnace 1, and the molten glass G1 is then allowed to overflowout from the top of the trough-shaped forming body 11 and fuse at thebottom thereof to form the glass ribbon G2 in a sheet form. Around thetrough-shaped forming body 11, a pair of the cooling rollers 12 isprovided. In view of the fact that the glass ribbon G2 is sandwichedbetween the cooling rollers 12 at its both edges, its both ends arecooled, so that the shrinkage in the width direction is minimized.

Next, the formed glass ribbon G2 is annealed in the annealing furnace 2to reduce the thermal shrinkage ratio thereof In the annealing furnace2, plural pairs of the guide rollers 22 are disposed in the verticaldirection and grasp the glass ribbon G2 to guide it downward. Inaddition, the inside of the annealing furnace 2 is sectioned into afirst annealing zone 231 corresponding to the first annealingtemperature region (from the (annealing point +200° C.) to the(annealing point +50° C.)), an intermediate annealing zone 232corresponding to the intermediate annealing temperature region (from the(annealing point +50° C.) to the annealing point), and a secondannealing zone 233 corresponding to the second annealing temperatureregion (from the annealing point to the (annealing point −100° C.)), andan output of each heater 21 is controlled such that the cooling rate inevery zone differs from each other.

In the cooling section 3 provided in a lower portion of the annealingfurnace 2, the glass ribbon G2 is cooled to substantially roomtemperature by means of natural cooling.

In the cutting section 4 provided just below the cooling section 3, theglass ribbon cooled to the vicinity of room temperature is cut into theglass sheet G3 having a prescribed dimension and conveyed into thesubsequent step.

Using the foregoing production equipment, a glass substrate having acomposition containing 60% of SiO₂, 15% of Al₂O₃, 10% of B₂O₃, 8% ofCaO, 5% of SrO, and 2% of BaO in terms of percentage by mass and havinga size of 500 mm×650 mm×100 μm in thickness (annealing point: 705° C.,strain point: 655° C.) was produced under two kinds of annealingconditions. The annealing condition (average cooling rate), the fictivetemperature, the thermal shrinkage ratio, the average surface roughnessRa, the sheet thickness difference, the residual stress value, and thewarpage value are shown in Table 1. Incidentally, in producing theforegoing glass, the respective zones were set such that the firstannealing temperature region was from 905 to 755° C., the intermediateannealing temperature region was from 755 to 705° C., and the secondannealing temperature region was 705 to 605° C.

Incidentally, the average cooling rate was computed based on thetemperature of the glass measured with a pyrometer.

TABLE 1 Example 1 Example 2 Example 3 Annealing condition (° C./min)First annealing zone 750 750 420 Intermediate annealing zone 150 530 380Second annealing zone 120 250 120 Fictive temperature (° C.) 760 760 750Average surface roughness Ra (nm) 0.2 0.2 0.2 Sheet thickness difference(nm) 15 15 20 Residual stress value (nm) 1.5 1.5 1.3 Warpage (μm) 70 7060 Thermal shrinkage ratio (ppm) 40 50 40

As is clear from the table, it is noted that when the average coolingrate in the first annealing temperature region is high, the sheetthickness difference becomes small, and when the average cooling rate inthe second annealing temperature region is low, the thermal shrinkageratio becomes small. In addition, in Examples 1 and 3, 100 pm-thickglass substrates having excellent surface quality and a thermalshrinkage ratio of 40 ppm were obtained.

Incidentally, the strain point and the annealing point were measuredbased on the method according to ASTM C336-71.

The fictive temperature was determined as follows. First of all, thesame glass piece as that in the foregoing thermal shrinkagedetermination was put into an electric furnace controlled at 705° C.,and after one hour, the glass piece was taken out of the electricfurnace and rapidly cooled on an aluminum plate, followed by measuring athermal shrinkage ratio thereof. The same treatment was carried out at725° C., 745° C., and 765° C., respectively, and a graph of arelationship between the treatment temperature and the thermal shrinkageratio was prepared. A heat treatment temperature at which the thermalshrinkage ratio was 0 ppm was determined from a linear approximate curveof this graph, and this was defined as the fictive temperature of glass.

The average surface roughness Ra was measured according to a method inconformity with the “FPD Glass Substrate Surface Roughness MeasurementMethod” in SEMI D7-94.

The residual stress value was measured using a stress meter,manufactured by Uniopt Corporation, Ltd. according to an opticalheterodyne method.

The warpage value was measured as follows. That is, a sample having asize of 550 mm×650 mm, as cut out from the center portion of the glasssubstrate, was measured with a glass substrate warpage measurementsystem, manufactured by Toshiba Corporation.

A value obtained by measuring thickness variation along an arbitraryline across a glass substrate using a laser type thickness measuringdevice, determining a maximum thickness and a minimum thickness of theglass substrate, and then subtracting a value of the minimum sheetthickness from a value of the maximum sheet thickness was defined as thesheet thickness difference.

The thermal shrinkage ratio was determined as follows. As shown in FIG.2( a), linear markings were given to predetermined sites of the glasssheet G3; and thereafter, as shown in FIG. 2( b), this glass sheet G3was broken vertically to markings M and divided into two glass sheetpieces G31 and G32. Then, only one glass sheet piece G31 was subjectedto a predetermined heat treatment (heating from ordinary temperature ata rate of 5° C./min, keeping at 450° C. for a holding time of 10 hours,and then cooling at a rate of 5° C./min). Thereafter, as shown in FIG.2( c), the heat-treated glass sheet piece G31 and the untreated glasssheet G32 were put in parallel, the both were fixed with an adhesivetape T, and a marking displacement was determined. The thermal shrinkageratio was calculated according to the following numerical formula 1.

$\begin{matrix}{S = {\frac{{\Delta \; {l_{1}({µm})}} + {\Delta \; {l_{2}({µm})}}}{l_{0}({mm})} \times 10^{3}\mspace{14mu} ({ppm})}} & {{Numerical}\mspace{14mu} {Formula}\mspace{14mu} 1}\end{matrix}$

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof

Incidentally, the present application is based on a Japanese patentapplication filed on Mar. 23, 2010 (Japanese Patent Application No.2010-65568) and a Japanese patent application filed on Mar. 8, 2011(Japanese Patent Application No. 2011-49763), the entire contents ofwhich are incorporated herein by reference. All references cited hereinare incorporated in their entirety.

INDUSTRIAL APPLICABILITY

The glass sheet produced according to the method of the presentinvention is suitable as a substrate for flat panel displays which arerequired to achieve thinning and weight reduction, such as liquidcrystal displays, organic EL displays, etc., and a substrate fordisplays which are required to have flexibility. Furthermore, thepresent invention can be used for new display applications requiring athin-film electric circuit, such as electronic paper, digital signage,etc.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1: Forming furnace

11: Trough-shaped forming body

12: Cooling roller

2: Annealing furnace

21: Heater

22: Guide roller

231: First annealing zone

232: Intermediate annealing zone

233: Second annealing zone

3: Cooling section

4: Cutting section

G1: Molten glass

G2: Glass ribbon

G3: Glass sheet

G31, G32: Glass sheet piece

M: Marking

T: Tape

1.-4. (canceled)
 5. A glass substrate having a sheet thickness of from10 to 200 μm, wherein a sheet thickness difference between a maximumsheet thickness and a minimum sheet thickness in the substrate is notmore than 30 μm.
 6. The glass substrate according to claim 5, wherein aresidual stress value thereof is not more than 2.5 nm.
 7. The glasssubstrate according to claim 5, wherein a warpage value thereof is notmore than 200 μm.
 8. The glass substrate according to claim 5, wherein athermal shrinkage ratio at the time of heating from ordinary temperatureat a rate of 5° C./min, keeping at 450° C. for 10 hours and then coolingat a rate of 5° C./min is less than 300 ppm.
 9. The glass substrateaccording to claim 5, wherein an average surface roughness Ra thereof isnot more than 0.3 nm.
 10. The glass substrate according to claim 5,which is composed of a glass comprising from 50 to 70% of SiO₂, from 10to 25% of Al₂O₃, from 1 to 15% of B₂O₃, from 0 to 10% of MgO, from 0 to15% of CaO, from 0 to 15% of SrO, from 0 to 15% of BaO, and from 0 to 5%of Na₂O in terms of percentage by mass.
 11. The glass substrateaccording to claim 5, which is used as a substrate for forming athin-film electric circuit.
 12. The glass substrate according to claim5, which is used as a substrate of a flexible display.